Access skin modifier radius data
Reading and writing these values from Python must happen in Object mode. I added a plane and deleted one vertex, added the skin modifier and decreased the radius of one vertex.

&gt;&gt;&gt; obj = bpy.data.objects['Plane']
&gt;&gt;&gt; for v in obj.data.skin_vertices[0].data:
...     print(v.radius[:])
...     
(0.25, 0.25)
(0.25, 0.25)
(0.08836718648672104, 0.08836718648672104)


Each element in skin_vertices[0].data has radius, use_loose, use_root to read and write. Radii can be any two element iterable (tuple, list).

import bpy

obj = bpy.data.objects['Plane']
for v in obj.data.skin_vertices[0].data:
    v.radius = 0.2, 1.2


I just realized that CtrlA lets you constrain the effect on radius per axis with ShiftX,Y.